This invention relates to automotive brake booster devices and more particularly to those of the type including a booster shell, a booster piston accommodated in the booster shell, a first working chamber defined in the booster shell by the booster piston on the front side thereof and continuously held in communication with a source of vacuum supply, a second working chamber defined in the booster shell by the booster piston on the rear side thereof and adapted to be selectively placed in communication with the first working chamber and the external atmosphere by a control valve, an input rod connected with a brake pedal and arranged opposite to the booster piston for movement toward and away from the latter, said control valve being arranged between the input rod and the booster piston and operable upon forward movement of the input rod to produce such a pressure differential between the first and second working chambers as to cause the booster piston to follow the movement of the input rod, and an output rod operatively connected with the booster piston for driving engagement with the operating piston of a brake master cylinder.
Conventionally, in this type of brake booster device, provision has been made to enable the master cylinder piston to restore its proper retracted position by arranging so that, when the booster input rod is in its full-retracted position, the booster output rod and the master cylinder piston are held with a small axial clearance or gap left therebetween which is sufficient to accommodate machining errors on the associated parts. Also, provision has been made to ensure that, in the released state of the booster device, the booster piston is maintained in its predetermined, retracted position, by arranging so that, in the booster state, the control valve is held in a normal state switched to place the second booster chamber in communication with the first booster chamber. In such booster device, when the input rod is advanced forwardly from its normal, retracted position, the control valve is switched from its normal, retracted state to its operative state to start the booster piston forwardly. On this occasion, however, the operation piston of the brake master cylinder remains inoperative until the booster output rod is brought into abutting engagement therewith. The distance of advancing movement of the booster input rod effected by this time represents a lost motion or an idle range of advancing stroke of the input rod which has no effect on the booster piston. This obviously calls for a corresponding considerable increase in initial, idle travel of the brake pedal, which is actuated with a substantial lever ratio, giving rise to an undesirable delay in operation of the brake master cylinder.